A wide variety of thermoplastic parts are formed by injection molding. The thermoplastic material is typically heated to an elevated temperature such that the material becomes substantially molten and will flow readily under pressure into the mold. To cure the thermoplastic material, it is cooled in the mold until it is sufficiently solidified.
Thermosets, such as rubber, can also be injection molded. However, these materials have different properties than thermoplastics and must be treated differently in the molding process. For instance, to injection mold rubber it is typically heated to a relatively low initial temperature (on the order of about 150.degree. F. to 200.degree. F.) so that is pliable, although not molten or readily flowable, and can be injected into a mold. Once injected into the mold, the rubber cures as a function of its temperature and the time held at an elevated temperature. For instance, rubber will cure relatively rapidly at higher temperatures and will require longer curing times at lower temperatures. Thus, rubber cures in a very different way than thermoplastics and hence, requires a different injection molding process which has its own parameters and presents problems different from those involved in injection molding thermoplastics.
Prior systems for injection molding of rubber have used a single block of steel with the desired passages or runners formed therein to communicate the main injection nozzle with secondary nozzles through which the rubber is injected into the mold. To form the runners in the single block of steel a plurality of holes must be drilled and portions thereof subsequently plugged to prevent leakage from the block. Careful machining of the block to provide the runners is necessary to avoid sharp comers in the passages or runners which may inhibit the flow of the rubber material and eventually cause blockages of the runners.
Furthermore, the injection block is typically at a sufficiently elevated temperature to insure that the rubber remains pliable and can be injected into the mold. When the molding process is interrupted or stopped for a duration, the rubber within the injection block may begin to cure and block the runners preventing subsequent injection molding operations. Blockage of the runners also occurs throughout use of the injection block and especially around comers or other non-linear portions of the runners wherein the flow of rubber through the runners may be inconsistent or inhibited and some of the rubber begins to cure in the runners eventually causing a blockage. With the injection block formed of a single piece of steel, it is extremely difficult to adequately clear a blockage from a runner and often such blockages must be machined from the block, such as by drilling, which can damage the surface of the runners and may thereby cause subsequent blockages. Thus, with an injection block formed from a single piece of steel it is labor intensive and expensive to initially form the runner system therein and plug the open ends, and it is subsequently difficult, labor intensive and expensive to remove blockages from them and they are thus, costly to maintain.
Attempts to form a suitable injection block for the injection molding of rubber from multiple flat plates secured together by fasteners such as cap screws, such as has been done for the injection molding of thermoplastics, have been unsuccessful because after a period of use the fasteners break and the plates are forced apart and damaged. This problem is not encountered with the injection molding of thermoplastics which tend to flow more uniformly through the runners when in their generally molten condition such that the metal to metal seal between adjacent plates is sufficient to prevent the plastic from leaking or creeping between the plates.